This fault is a huge worry now because of warningsthat the “Big One”—an earthquake of magnitude 6.7 or greater—is overdue. Such a large quake could cause vast destruction in some of the nation’s most populous cities, such as Los Angeles.

Now new findings indicate that water being drawn out of the large aquifer that sits beneath California’s San Joaquin Valley—located in the center of the state—is changing stresses on the San Andreas fault. Scientists aren’t saying that removing water will cause the Big One anytime soon, but they do note that groundwater depletion could be responsible for some of the changes seen in the frequency of small earthquakes in the region. So report Colin B. Amos of Western Washington University in Bellingham and colleagues today in Nature.

From the air, the San Andreas fault can be clearly seen where it crosses the Carrizo Plain in California.
(Ikluft/Wikimedia Commons)

The researchers analyzed data from global positioning system stations located across a wide swath of California and Nevada. The instruments are so precise that they can detect changes in movement of the land of less than a millimeter per year.

Those measurements showed that the region that surrounds the San Joaquin Valley is rising by about one to three millimeters per year. That movement is how the earth is compensating for subsidence in the valley itself. The subsidence is occurring because of the removal of water from the aquifer beneath. About 160 cubic kilometers (42 trillion gallons) of water have been removed over the last 150 years. Much of that has gone to irrigate the thousands of acres of farm fields that feed much of the United States.

The uplift in this area isn’t much, but it’s enough to “increase the sliding on the San Andreas fault system” by changing the loads burdening the fault, notes Paul Lundgren, of the California Institute of Technology in Pasadena, in an accompanying News & Views article. “Over the long-term (a 100-year timescale), the unloading generated by constant water extraction…enhances the accumulation of stress between the occurrence of large earthquakes, bringing faults such as the San Andreas closer to failure.”

The study’s results “suggest that human activity may give rise to a gradual increase in the rate of earthquake occurrence, as suggested by earthquake catalogues in central California,” Amos and colleagues write.

This is not the first time that scientists have linked groundwater extraction to earthquakes. In 2012, for instance, researchers reported that the unusually shallow magnitude-5.1 earthquake in Lorca, Spain, in 2011 had been triggered by the removal of water from the ground. And geologists have known for some time that the injection of fluid into the ground—such as for the disposal of fracking wastewater—can trigger quakes.

But what’s worrying here is that the need for California’s groundwater is only going to increase. The population is growing. “Future scenarios for groundwater in California suggest increasing demand for agricultural, urban and environmental use,” Amos and colleagues note. Place that the context of climate change, which “will probably exacerbate the stress on this resource through altered precipitation patterns, more frequent droughts, earlier snowmelt, larger floods and increasing temperatures and evapotranspiration,” they write.